I often get asked how to build the shelter if there's no convenient hill-side to burrow into.
Well, there are three common approaches that are used that still make use of a layer of dirt to provide storm protection. Here are some simple illustrations:
The biggest problem with these is drainage; with Full Berm, or going into a hillside like I did, worst case scenario the water will just drain out of your shelter through the door. You might have half an inch of water, but the rest is going to leave. Once the floor of your shelter is below ground level you have to start getting a lot more careful with drainage unless you want to run out to your shelter and find you have a cistern instead.
When it comes to berming over your shelter you have several option; you can grade the soil so it naturally holds it's angle and shape, or you could use some sort of earth retaining wall. Personally, I kind of like the idea of gabion baskets with rocks in them, but that would be a lot of work.
Another option is to build the entire structure above ground and finish it out with siding and a proper roof so it looks just like a little utility shed. I'd be sure to run rebar down through EVERY core and I'd probably make the roof somewhat thicker, but this is basically how above-ground tornado proof structures are built.
So, don't be discouraged just because you don't have a sloping hillside to dig into. There are still other ways to build a concrete storm shelter.